1. Field of the Invention
The invention relates to a method and apparatus for determining the moisture content of materials such as particulate materials, soils or solid materials such as concrete.
2. Description of the Prior Art
Doerr-weighing method
A classical method for determining moisture content resides in determining the weight of a sample before and after drying and, for many goods, this is standard procedure for individual evaluations. Automation, however, requires continuously operating measuring procedures which do not incur any substantial delays. The need for taking samples is also a disadvantage of this method.
Calcium carbide method
A large number of chemical moisture content determination methods are available. In one well-known method of this type, a sample, for example of sand, is mixed with calcium carbide in a pressurizable vessel. The pressure resulting from the formation of acetylene is then used as a measure for the moisture content of the sand. It is pointed out, however, that this method is neither continuous nor is it particularly simple and inexpensive and it also requires sample taking.
Karl-Fischer method
This method which utilizes an iodometric reaction is the most common of the chemical moisture content determination methods. It permits determination of trace contents of only 0.1 mg in the solvent extract of a sample. In situ utilization of this method however is not possible.
Electric conductivity measuring method
Measuring the moisture content by determination of the electric conductivity of a material is generally the preferred method since it is inexpensive and requires only simple apparatus and since it is continuous and practically free of delays. The measuring range with conductivity measurement corresponds to the hygroscopic range of the materials which are non-conductive in a dry state. For very dry materials the conductivity determination is not appropriate because of the high resistance and for very wet materials this method is not suitable because of large electrolyte influences.
Neutron measurements
Neutron radiation is greatly slowed by light water atoms whereas it can penetrate most heavy elements with relative ease. If directed through a material, part of the fast neutron, radiation is scattered and slowed down depending on the water content or rather the hydrogen content of the material. A counter tube or a scintillation counter determines a representative portion of the slow neutrons so produced and forms a measure for the moisture content of the material based on a certain volume, utilizing an impulse rate with a time constant of about 1 minute. It is considered to be particularly advantageous that with this method the measured value is integrated for a large volume of material, that is, a good average value is obtained based on a large volume of material. However the method cannot be used if the materials have varying hydrogen contents bound in compounds. It is particularly disadvantageous that a neutron radiation generator has to be utilized.
Dielectric measurement
In contrast to the method utilizing electric conductivity, this method can also be used for liquids, for very dry and very wet materials. It detects also moisture which is enclosed in insulated areas and it averages the water content in contrast to the electric conductivity measurement which, during measurement over a wide track, will consider mainly wet pockets. With an appropriate measuring capacity, dielectric measurements permit measuring far into the wet area, that is, materials with very high moisture content.
Since the present invention relates to a type of dielectric measuring method this state of the art will now be considered in greater detail.
In principle there is a monotonic clear connection between the dielectric constant .epsilon. of a medium and its moisture content. The problem of measuring the moisture content is therefore reduced to the problem of measuring the dielectric constant .epsilon.. In the area of concern two main methods have been developed:
i. Condenser Method:
Two plates are provided in a predetermined spatial arrangement with the material to be measured disposed between the plates, and the dielectric constant .epsilon. is determined by way of the capacity of the arrangement. However, with this arrangement, measuring problems are encountered and the size of the condenser plates may become prohibitively large.
ii. Echo Method:
Here a conductor is provided in a long open-ended geometric arrangement. The conductor is electrically connected so that a short pulse can be applied of a duration which is shorter than the time required for the return of the echo from the end of the conductor. If the conductor is in the form of two parallel rods which are inserted into the material to be measured, the dielectric constant can be determined from the travel time of the echo and the moisture content can be determined from the dielectric constant. This method however has encountered problems because of the short time lag between signal application and echo return (ca-10 ns). The excitation pulses should therefore be substantially shorter. In order to permit division of the time between pulse application and return of the echo into a reasonable number of time increments, the pulse duration should be in the area of 1/100 ns. Apparatus known in the art sense the voltage at the beginning of the conductor as a transient (partial sampling method) and analyze the voltage curve manually or automatically. This however requires a substantial amount of equipment particularly because of the needed short duration and accurate repeatability of the signals and their periodic follow-ups required for the recording procedure.
It is the object of the present invention to provide a material moisture content sensing apparatus which is relatively inexpensive through utilization of readily available electronic circuit elements but which nevertheless offers high measurement accuracy and automatic material moisture determination and which does not require correction adjustment during use.